(A) Field of the Invention
The present invention relates to a leadframe for a leadless package, structure and manufacturing method using the same, and more particularly, to the structure of a quad flat non-leaded (QFN) package, a leadframe thereof and a manufacturing method using the same.
(B) Description of the Related Art
To meet electronic devices' requirements for lightness, thinness and small size, QFN packaging has overtaken the traditional leadframe packaging method to replace the more expensive wafer level chip size packaging (CSP). While CSP scales down the package size so that it is the same as the die size, it needs a fine-pitch ball grid array to act as pins, which makes the manufacturing process more complicated. In contrast, QFN packaging is not only with a small size, a low cost and a high manufacturing yield, it is also a high speed and power managing circuit device with a better coplanarity and high heat dissipation capacity. In addition, the QFN packaging does not need to extend the pins from its sides, and therefore its electrical performance is better than the traditional leaded packaging which needs to extend several pins from its sides, and sometime these pins serving as an inductance or a capacitor produce a lot of noise when applied to high frequency systems.
Furthermore, the exposed lead frame pad of QFN packaging can serve as a direct heat dissipation path, which enables the package to exhibit a better heat dissipation capacity. Normally, such a thermal pad is directly soldered on the circuit board, and the thermal via in the circuit board brings the surplus heat to the copper foil connected to the ground, so additional cooling fins can be saved.
FIG. 1 shows the cross-sectional schematic view of a prior art example of a QFN package structure. A QFN packaging element 80 includes a lead-frame 81, a die 82, an adhesive 83, a plurality of leads 84 and a molding material 85, wherein the die 82 is mounted on a die pad 811 of the lead frame 81 by means of the adhesive 83, and the plurality of leads 84 electrically connect the die 82 to a plurality of pins 812 of the lead frame 81. The die 82, the leads 84 and the leadframe 81 are covered with the molding material 85, while the molding material 85 does not cover the lower surfaces of the die pad 811 and the pins 812. The uncovered parts of the surface of the pins 812 act as the contacts of the surface mounting. The uncovered part of the surface of the die pad 811 can dissipate heat directly to outside and therefore replace the heat sink of the prior art serving the same function. However, the die pad 811 is disposed on the center of the pins 812, and needs to be kept at a proper distance from the surrounding pins 812, so the package size is limited. In view of above, because the heat dissipation efficiency is closely related to the package size, if the uncovered area of the surface of the die pad 811 can be increased, it will help solve the more and more serious problem of heat dissipation that a multiple function die especially faces.